The use of an ion-exchange membrane as a support for facilitated transport offers several important advantages over other methods of separation: (1) in an ion-exchange membrane, the complexing agent is held in the membrane by electrostatic forces and cannot be leached out; (2) the concentration of complexing agent in the membrane is determined by ion-exchange site density and not physical solubility. The ion-exchange site density is normally much larger than the physical solubility; (3) the high charge density in the vicinity of the ion-exchange sites protects the complexing agent from redox reactions and extends its useful lifetime; and (4) solvent loss problems are reduced. The complexing agent is not removed if the solvent phase is removed. The membrane can be resolvated without a reduction in subsequent performance.
Ionomers are polymeric materials containing ionic groups. Most of the research effort on ionomers has focused on only a small number of materials such as ethylenes, styrenes, rubbers, and fluorocarbon-based ionomers. Because of a high water permeability and cation selectivity, fluorocarbon-based ionomers have been used as ion-exchange membranes (Kyu (1985) in Materials Science of Synthetic Membranes, D. R. Lloyd, ed. (American Chemical Society, Washington, D.C.), pp. 365-405). Perfluorinated ion-exchange membranes are derived from copolymers of tetrafluoroethylene (TFE) and a perfluorovinyl ether terminated by a sulfonyl fluoride group. Examples of perfluorinated ion-exchange membranes include Nafion.RTM. (E. I. du Pont de Nemours), Femion.RTM. (Asahi Glass Co., Ltd.), and Neosepta-F.RTM. (Tokuyama Soda Co. Industry Company) (Kyu (1985) supra).
Perfluorinated ionomer membranes are characterized by high chemical and thermal stability and strength, high water permeability, and cation permselectivity. These characteristics make them ideal membranes in many separation applications (Kyu (1985) supra). Perfluorinated ionomer membranes are widely used in chlor-alkali cells, water electrolyzers, batteries, and fuel cells (Kipling (1982) in Perfluorinated Ionomer Membranes, A. Eisenberg and H. L. Yeager, eds. (American Chemical Society, Washington, D.C.) pp. 475-487). They have been used in a number of nonelectrochemical applications including chemical separations, organic syntheses, and catalytic systems (Moore and Martin (1988) Macromolecules 21:1334-1339).
Perfluorinated ionomer membranes contain carbon backbones made up of fluorocarbon chains. Ionic groups are connected to the backbone through side chains. The side chains can terminate in a variety of ionic groups, including sulfonic acid, carboxylic acid, sulfonium, or quaternary ammonium (Fujimura et al. (1981) Macromolecules 14:1309-1315). In the case of an acidic side chain, the locations of the anions are fixed whereas the cation can transport through the polymer.
Nafion.RTM., manufactured by E. I. du Pont de Nemours, is a cation exchange membrane that consists of a fluorocarbon backbone with fluorocarbon sidechains (Besso and Eisenberg (1981) in Proceedings of the Symposium on Ion Exchange Transport and Interfacial Properties, R. S. Yeo and R. P. Buck, eds. (Electrochemical Society, Pennington, N.J.), pp. 197-209; Yeager and Eisenberg (1982) in Perfluorinated Ionomer Membranes, A. Eisenberg and H. L. Yeager, eds. (American Chemical Society, Washington, D.C.), pp. 1-6). Thin Nafion.RTM. films are particularly effective for the selective passage of water, cations, and water-soluble molecules, and as supports for facilitated transport separations. A primary application for Nafion.RTM. membranes is in the chlor-alkali industry (Yeager and Steck (1981) J. Electrochem. Soc. 128:1880-1884; Yeager and Eisenberg (1982) supra). Nafion.RTM. has also been used as a solid polymer electrolyte in an experimental photoelectrochemical cell (Sammells and Schmidt (1985) J. Electrochem. Soc. 132:520-522), and in a zinc bromide (ZnBr2) battery (Lim et al. (1977) J. Electrochem. Soc. 124:1154-1157; Will (1979) J. Electrochem. Soc. 126:36-42). A major limitation to more widespread use of Nafion.RTM. is its high resistance to mass transport through the polymer structure.
Because of the technological importance of the perfluorinated ionomers, their microscopic structure and the relationship of structure to membrane transport properties have been extensively studied (Yeager and Steck (1981) supra; Yeager et al. (1982) J. Electrochem. Soc. 129:85-89; Yeo (1983) J. Electrochem. Soc. 130:533-538; Fales et al. (1986) in Proceedings of the Symposium on Engineering of Industrial Electrolytic Processes (Electrochemical Society, Pennington, N.J.), pp. 203-218; Sakai et al. (1986) J. Electrochemical Soc. 133:88-92; Fujimura et al. (1981) supra; Gierke et al. (1981) J. Polym. Sci. Polym. Phys. Ed. 19:1687-1704). Among the techniques used to probe different aspects of the structural features of perfluorinated ionomer membranes are small angle X-ray scattering, small angle neutron scattering, quasi-elastic neutron scattering, infrared nuclear magnetic resonance, and Mossbauer spectroscopy (Yeager and Eisenberg (1982) supra).
Small angle X-ray scattering (SAXS) is a technique for studying material structural features that are on the order of a few nanometers in size (Kratky (1982) in Small Angle X-Ray Scattering, O. Glatter and O. Kratky, eds. (Academic Press, New York), Chapter 1). Any material with at least two phases having different electron densities will give a scattering pattern which is dependent on the shape and dimensions of the different regions (Porod (1982) in Small Angle X-ray Scattering, O. Glatter and O. Kratky, eds. (Academic Press, New York), Chapter 2). Electron density is defined as the moles of electrons per unit volume where each electron is a possible scattering sight for X-rays.
For perfluorinated ionomers, a significant electron density difference between the ionic cluster and surrounding fluorocarbon region has been estimated (Roche et al. (1981) J. Polym. Sci. Polym. Phys. Ed. 19:1-11), supporting the idea that the PFSA microstructure contains at least two separate domains (Gierke et al. (1982) in Perfluorinated Ionomer Membranes (American Chemical Society, Washington, D.C.) pp. 195-216). One domain is composed primarily of the hydrophobic fluorocarbon backbone. A second domain contains ion-exchange sites that are part of the polymer backbone, called ionic clusters. A third domain is the interfacial region containing some side chain materials, small amounts of water, some sulfonate sites with cations and a relatively large fractional void volume.
Ionic clusters are formed by the grouping of ionic sulfonate groups within the polymer (Roche et al. (1981) supra; Yeo and Cheng (1986) J. Appl. Polym. Sci. 32:5733-5741) and are small regions where ionic chemistry dominates. The average size of ionic clusters within Nafion.RTM. membranes has been estimated to be on the order of 40-50 .ANG. (Gierke et al. (1981) supra; Kyu (1985) supra). The ionic clusters may be linked by channels forming a network throughout the membrane. As with all ionomers, the sidechains are permanently attached to the polymer backbone at random intervals. The side chains are relatively immobile, but the counterion is free to move. Counterion motion makes these polymers ionic conductors. Specific molecules, which are polar or charged, can easily diffuse through a film of PFSA by way of the ionic cluster channels.
Efforts have been made to modify the basic Nafion.RTM. homogeneous polymer film to produce materials with special characteristics, including lamination of fabric to the polymer film to increase its strength, composite membranes made up of layers of different equivalent weights of polymer film laminated together to increase anion rejection, and surface treatment to improve hydroxide ion rejection (Yeager and Eisenberg (1982) supra).
The literature also contains numerous reports of structural and morphological modifications to PFSA polymers and film-casting strategies that attempt to improve the productivity of cast membranes (Moore and Martin (1988) supra and (1986) Anal. Chem. 58:2569; Liu and Martin (1990) J. Electrochem. Soc. 137:3114; Gebel et al. (1987) Macromolecules 20:1425-1428; Heaney and Pellegrino (1989) J. Memb. Sci. 47:143-161). For example, Dow Chemical Corporation has a commercially available polyperfluorosulfonic acid (PFSA) material of substantially lower equivalent weight than Nafion.RTM. (U.S. Pat. No. 4,417,969 of Ezzell et al., issued Nov. 29, 1983). Equivalent weight is defined as the grams of polymer per one mole of ion exchange sites when the ionomer is in the acid form and dry (Yeager and Eisenberg (1982) supra). Low equivalent weight indicates a high density of ion-exchange sites per unit mass and often correlates well with lower mass transfer resistance (Gierke and Hsu (1982) supra; Yeo (1982) supra). Another report describes a procedure of heat treating Nafion.RTM. films that results in significantly increased permeability (Pellegrino et al. (1988) Gas Sep. and Purif. 2:126-130). Casting strategies such as forming very thin PFSA films on hollow fibers and other substrates to take advantage of more effective geometries also exist (U.S. Pat. No. 4,469,744 of Grot et al., issued Sep. 4, 1984). Attempts to increase the productivity of PFSA films often results in many of the advantageous chemical and physical properties being compromised.
Two attempts to provide improved separator materials have included the use of a surfactant species in the polymer formation process. U.S. Pat. No. 4,289,600, issued Sep. 15, 1981, to Lazarz et al. describes a microporous electrolytic cell separator produced from a mixture of polytetrafluoroethylene (PTFE), a particulate pore-forming material, and an organic fluorinated surfactant. The surfactant is added up to 50% by weight in an isopropanol-water solution to aid the blending of the PTFE and pore-forming material by lowering surface tension. However, the surfactant of the Lazarz et al. membrane is not incorporated into the polymer structure nor does it alter the membrane microstructure. Further, the surfactant does not improve membrane transport characteristics since the permeability of the Lazarz membrane results from addition of pore-forming particulate material.
U.S. Pat. No. 4,741,744, issued May 3, 1988 to Wu et al. also describes a PFSA membrane with improved permeability characteristics produced from perfluorinated polymers containing pendant hydrated metal ionomer moieties. The Wu et al. membrane is produced by an emulsion polymerization of one or two types of monomers, a free radical initiator, a buffer and a fluorinated surfactant. The surfactant is used to aid in the polymerization process. The polymer formed is used to make membranes by a multitude of techniques, including casting films from solutions. The role of the surfactant in the Wu membrane specifically functions to form micelles in which polymerization takes place and to stabilize the polymer emulsion in the latex form throughout the reaction rather than to alter membrane microstructure so as to enhance membrane transport characteristics.
The present invention describes the production of improved ionomer membranes from PFSA solutions that retain the desirable characteristics of high mechanical strength, high hydrophilicity, high solvent and temperature resistance, and high crystallinity, while having improved transport properties relative to membranes described in the prior art. This improvement is achieved by the addition of a surfactant species to the PFSA polymer solution prior to casting the polymer film, resulting in a membrane with a measurably altered membrane microstructure and improved transport characteristics.
The improved membrane characteristics of the present invention may result from a number of possible routes. The presence of surfactant in the polymer solution may alter the internal microscopic structure of the membrane film, resulting in an improved polymer film morphology. The altered film morphology may be maintained with or without the persistant inclusion of the surfactant in the final membrane. If the surfactant persists in the final film, an improvement in the transport properties may result by interactions which include the surfactant. These include more ion-exchange sites for solvent, carriers, counterions, and transporting solutes.